Passive Intermodulation (PIM) is a form of interference that occurs in a wireless communication node when that node simultaneously transmits signals at multiple frequencies through passive devices. P. L. Liu, “Passive Intermodulation Interference in Communication Systems,” Electronics & Communication Engineering Journal, Vol. 2, No. 3, 1990, pp. 109-118. Such passive devices may include, for instance, cables, connectors, antennas, and other such devices included in a node's transmit path. Many wireless communication nodes include such passive devices, but PIM interference proves particularly pronounced in nodes that transmit at high power.
The mechanisms that cause PIM interference are complex, but they can generally be divided into two major categories. The first category includes metal-to-metal contacts, including imperfect metal contacts, oxidized or contaminated contact surfaces, dissimilar metals in contact, and so on. D. E. Foster, “A new form of interference—external cross modulation,” RCA Review, 1:18-25, 1937. The second category includes material nonlinearity, including magnetic materials in the signal path, temperature variation, etc. Y.-S. Wu, W. H. Ku, and J. E. Erickson, “A study of nonlinearities and intermodulation characteristics of 3-port distributed circulators,” IEEE Transactions on Microwave Theory and Techniques, 24:69-77, 1976.
Regardless of the particular mechanism causing PIM interference, such interference degrades receiver sensitivity. Consider the example in FIGS. 1A-1B. In this simple example, a wireless communication node simultaneously transmits two signals at different frequencies f1 and f2 through a passive device 2 with a non-linear response. Although the signals are specifically transmitted in the node's transmit (TX) band 4, the transmission generates PIM that spreads over the frequency spectrum. Of particular concern, some components of PIM leak or couple into the node's receive (RX) band 6. These PIM components appear as interference to the node's receiver.
In more detail, FIG. 1B shows that the odd-order PIM components remain close to the TX band 4 and pose a particular threat to the RX band 6. The 3rd-order components (e.g., 2f1−f2 and 2f2−f1), in particular, have the highest possibility of coupling into the RX band 6, especially if f1 and f2 are separated by a large frequency gap. Moreover, these 3rd-order components have powers that are significantly higher than that of other odd-order components.
PIM components do not usually couple into the receive band in this manner in 3rd generation (3G) wideband communication systems such as the Universal Terrestrial Radio Access Network (UTRAN). This is because these systems use limited Radio Frequency (RF) bandwidth compared to the spacing between transmit and receive bands, so that third-order components of transmitter signals in particular fall outside the receive band. However, PIM components do couple directly into the receive band in other systems. Because these PIM components cannot be suppressed by filtering, the interference must be addressed in other ways.
PIM interference is addressed in narrowband communication systems, such as Global System for Mobile communications (GSM), by band planning and frequency hopping. H. Jung and O. K. Tonguz, “Random spacing channel assignment to reduce the nonlinear intermodulation distortion in cellular mobile communications,” IEEE Transactions on Vehicular Technology, Vol. 48, No. 5, 1999. This simple technique, however, does not adequately address PIM interference in communication systems that use very wide bandwidth, such as Long Term Evolution (LTE)/LTE Advanced systems, or that use techniques involving multiple frequency bands (e.g., Multi-Standard Radio in Non-Continuous spectrum (MSR-NC) and LTE Carrier Aggregation (CA) systems). 3rd Generation Partnership Project (3GPP) R4-111321, “On Passive Intermodulation (PIM) for MSR-NC,” Ericsson, February 2011. Indeed, band planning cannot provide enough isolation in these systems to prevent PIM components from coupling into the receive band.
Several alternatives have nonetheless developed in an attempt to address PIM interference. In one approach, the passive devices themselves are designed and manufactured so that they cause less pronounced PIM interference. High costs and design constraints, however, limit the practicality of this approach. Moreover, other factors such as a predominant use of re-cycled metals readily inhibit the performance gains that can be achieved with the approach.
In another approach, engineered PIM sources are added into the node's transmit path. These engineered PIM sources are designed to statically compensate for PIM generated by passive devices. Henrie, A. Christianson, and W. J. Chappell, “Cancellation of Passive Intermodulation Distortion in Microwave Networks,” in European Microwave Conference, Amsterdam, The Netherlands, 2008. This approach, however, requires additional hardware that is prohibitive in current communication systems. Even more problematic is that this approach fails to adequately address PIM because the engineered sources cannot dynamically track and compensate for the PIM interference, which varies over time.